The invention relates to an installation for the exploration of the subsoil with the aid of transient electromagnetic measurements.
The invention relates specifically to the data collection of transient electromagnetic signals, which are generated in the subsoil to be explored by feeding through a grounded dipole.
If such signals are to supply usable information concerning the structure of the subsoil, the always present interference sources which distort the signals must be avoided on the one hand, whereby, above all, alternating currents and radio signals can disturb the measuring results. On the other hand, it is necessary to set up several remote interspaced measuring points. The erratic changes in the transmitting current diffuse as transients in the subsoil and generate there expanding induction currents. The recording units contain measuring points which each measure different components of the electric and magnetic fields, originating from the induction currents. The invention picks up from a previously known installation of this type. In this installation, the measuring data are processed for the purpose of obtaining a most favorable ratio possible of measuring data and interference data. In the known installation, relay of the measuring data and the recording units into a central unit is required in the original form of the data. This requires that the measuring data are conveyed on separate channels and that they are digitized in the central unit. Because only maximum eight channels can be set up, the number of recording units is therefor limited (U.S. Pat. Nos. 3,737,768, and 4,535,293). Because the relay lines from the recording units to the central unit have an offset of up to 8 km depending on the requirements of the individual case, mixing in of interference signals cannot be avoided.
It is the objective of the invention to make a more efficient installation, the general construction of which has been described in the introduction, whereby especially the interference signals should be substantially eliminated. The invention solves the objective by locally digitizing and storing at each recording unit, the values that are measured at the recording unit and subsequently transmitting the stored data from each recording unit through a telemetric link to a central processing or analyzing unit, the stored data being transmitted from one recording unit at a time and in a preselected sequence in other aspects of the invention the stored data may be called up from each recording unit in a block and transmitted to and stored in the central unit. The digitizing of the measured values at each recording unit may be processed by a yield point amplifier. Automatic drift correction processing may also be applied to the measured values obtained at each recording unit. And, each recording unit may be provided with a microprocessor to process, store, and relay the data measured at the receiving unit and to test and control the status of the receiving unit.
According to the invention, the analog measuring values obtained by the measuring points via sensors are immediately digitized in the allocated recording unit, whereby the transmission path between the sensors picking up the measuring values and the digitizer can be kept very short. Transmission of the digitized signals occurs exclusively via telemetric link (line), whereby the number of recording units can be increased practically at random. A special advantage of the invention lies thereby in the fact that the frequency range from 0.01 to 0.3 kHz is not subject to any basic restriction due to a number of channels. The installation according to the invention can therefor carry out measurements without loss of quality with widely spaced out measuring points in a territory with strong electric interference.
The signals which are stored and digitized at the recording points are suitably called up individually and relayed to the central station via telemetric digital link. This embodiment of the invention permits calling up the data blocks as needed or in preset time sequence and to place them into the data bank of the central unit. Interpretation of the stored signals can subsequently follow, according to which a quantitative evaluation of the distribution of the electric resistances in the subsoil is possible, whereby the depth area explored can vary between a few meters and several kilometers.
Processing of the analog signals in the recording unit is not a prerequisite of the invention, but can occur, however, by processing with a yield point amplifier to digitize the measured values at each recording unit, for example, if this should be required in the individual case or if the user of the installation wishes this. Processing in these cases includes automatic drift correction in order to prevent the measurements from following a distortion path. Because the components of the electric and magnetic fields originating from the induction currents received at the measuring points are used, the vertical components of the magnetic field or their temporal derivative as well as, in addition, one or several components of the electric field are generally measured. This permits an interpretation with the aid of the Maxwell equations which leads to the above described quantitative evaluation of the electric resistances in the subsoil.
Furthermore, the invention enables the monitoring and control of the recording units, i.e. of their electronic analog components with a microprocessor installed at each recording unit.
Below, the invention is further described using an embodiment example, which is represented in the drawing. Shown are